The long-term objectives of this application are the following. The first goal is to understand the effects of cholesterol feeding and dietary fat saturation on the basic process of biliary sterol secretion and also upon t e contribution of lipoprotein to this event. The second goal is to determine whether high density lipoprotein (HDL) free cholesterol is preferred to oth r sources of cholesterol for biliary sterol secretion and whether biliary sterol secretion is affected by plasma HDL cholesterol concentrations. The studies proposed will use the African green monkey because this primate has several similarities to humans in terms of cholesterol metabolism. The dietary perturbations employed will take advantage of the greater occurence of cholesterol gallstones seen in this species when animals are fed diets with increasing amounts of cholesterol and polyunsaturated fat. These stud s should therefore provide valuable information regarding mechanisms of cholesterol gallstone disease. The specific aims of this proposal are the following: (1) To measure the effects of cholesterol feeding and dietary fat saturation on invivo biliary sterol secretion, bile acid pool sizes and maximal bile acid synthesis rate . This aim will be accomplished through the use of monkeys in which biliary cannulae have been surgically placed. Measurements will be compared betwee animals on three different diets: LO CHOL-SAT diet (containing 0.035 mg cholesterol/kcal and 35% of calories as predominantly saturated fat, P/S=0.38); HI CHOL-SAT diet (containing 0.80 mg cholesterol/kcal with 35% o calories as predominantly saturated fat, P/S=0.38); HI CHOL-POLY diet (containing 0.80 mg cholesterol/kcal and 35% of calories as predominantly polyunsaturated fat, P/S=2.3). (2) To compare the delivery of cholesterol to bile in vitro from HDL and other lipoprotein sources (LDL and VLDL) and determine wehter cholesterol feeding and/or degree of dietary fat saturatio affects these processes. These goals will be accomplished by performing isolated liver perfusion in livers from animals fed the diets noted above. Cholesterol radiolabeled HDL and cholesterol radiolabeled LDL or VLDL will be injected during separate time periods during the same experiment and the accumulation of radiolabel in liver and bile will be measured and compared between lipoprotein type and type of diet. (3) To determine if delivery of cholesterol to bile can be altered by plasma perfusate HDL concentrations. This will be accomplished by measuring and comparing the amounts of sterol mass and cholesterol radiolabel from HDL accumulated in bile during isolate liver perfusion when different amounts o f HDL are added to the perfusate. Amounts of HDL mass will be added to the perfusate so as to approximate a 0 - 100% range of normal plasma HDL cholesterol concentrations in African gree monkeys. (4) To assess hepatic cholesterol synthesis in animals fed the three diets noted above and to compare these values to biliary lipid secretion rates in order to estimate the maximal amount of biliary lipid th t could be secreted from newly synthesized cholesterol. These measurements will be made using [3H] water to assess synthesis during a one hour liver perfusion.